This invention relates to static dissipative polymers containing carbonyl groups in the polymer backbone.
With the recent proliferation of electronic equipment there has developed a rapidly growing market for static dissipative packaging materials. Many electronic devices, including printed circuits and microchips, are extremely vulnerable to damage from static discharges of as little as 50 volts. Since static discharges of up to 10,000 volts are commonly encountered during the normal handling of these devices, it is necessary to protect them from static discharges during their manufacture, transportation and use.
In transporting electronic devices, it is desirable to enclose the device in a packaging material which is lightweight, strong, easily fabricated into a configuration which is adapted to the size and shape of the electronic device, and which will protect the device from harmful static discharges. Plastics, and polyolefins in particular, possess all of the foregoing characteristics except the ability to provide static protection. Accordingly, much effort has gone into developing a suitable means for rendering polyolefins sufficiently static dissipative to use in packaging electronic devices.
One method is to laminate the polyolefin to a metal layer, or to employ one or more metal layers in a laminate structure. This method provides excellent static dissipative behavior, but is relatively expensive and not adaptable to a wide variety of uses. In particular, metalized polyolefins are very useful for packaging small components, but are often too difficult to fabricate to be useful packaging materials for larger devices, such as assembled electronic equipment. Moreover, having a metal in contact with sensitive electronics is undesirable since the metal is too conductive, and can conduct electrical charges to the electronic device rather than away from it. Also, a bimetallic effect is often seen between the metals in the electronic device and the metalized plastic.
Another approach is to render a polyolefin static dissipative by using a conductive additive. Various materials have been used for this purpose, including fillers such as carbon black, graphite and metallic fibers. Although these materials impart static dissipative behavior to the polymer, they also radically change its physical characteristics, rendering it unsuitable for fabrication into many applications. These fillers have the potential drawback of introducing contamination to the electronic device. In addition, the metallic fibers have the same limitations as metalized plastics.
Yet another approach is to incorporate an amine, humectant or surfactant compound into the polymer. These materials operate by exuding to the surface of the polymer, where they absorb atmospheric moisture to form an electrolyte microlayer. This microlayer is sufficiently conductive to render the polymer static dissipative. Four major problems are encountered with this approach. Since the static dissipative agents are on the surface, they are subject to being rubbed off during handling. In this manner, the static dissipative effect is reduced or destroyed until more of the amine, humectant or surfactant can migrate to the surface. Further, since the static dissipative agent is being continually removed, the polymer will eventually lose its static dissipative properties. In addition, these static dissipative agents depend on a humid environment for effective operation. Thus, their static dissipative behavior will vary according to the local relative humidity, and will be minimal in arid environments. Finally, these static dissipative agents are sometimes corrosive or are potential contaminants.
Accordingly, it would be desirable to provide a static dissipative polymer which has excellent static dissipative properties which are not significantly dependent on local humidity, are stable over time, and which has physical properties which permit it to be used for a variety of packaging and other static control applications.